Use cache.julialang.org to store ML models

It's annoying that when third party servers go down our tests break.
Let's at least make sure that if our tests break due to server outages
it's our fault.

src/data/cmudict.jl

@@ -10,7 +10,7 @@ function load()
   isdir(deps("cmudict")) && return
   mkpath(deps("cmudict"))
   for x in ["", ".phones", ".symbols"]
-    download("http://svn.code.sf.net/p/cmusphinx/code/trunk/cmudict/cmudict-$version$x",
+    download("https://cache.julialang.org/http://svn.code.sf.net/p/cmusphinx/code/trunk/cmudict/cmudict-$version$x",
              deps("cmudict", "cmudict$x"))
   end
 end

src/data/mnist.jl

@@ -14,7 +14,7 @@ function load()
                  "t10k-images-idx3-ubyte",
                  "t10k-labels-idx1-ubyte"]
       isfile(file) && continue
-      download("http://yann.lecun.com/exdb/mnist/$file.gz", "$file.gz")
+      download("https://cache.julialang.org/http://yann.lecun.com/exdb/mnist/$file.gz", "$file.gz")
       open(file, "w") do io
         write(io, GZip.open(read, "$file.gz"))
       end

src/data/sentiment.jl

@@ -5,7 +5,7 @@ using ..Data: deps
 
 function load()
   isfile(deps("sentiment.zip")) ||
-    download("https://nlp.stanford.edu/sentiment/trainDevTestTrees_PTB.zip",
+    download("https://cache.julialang.org/https://nlp.stanford.edu/sentiment/trainDevTestTrees_PTB.zip",
              deps("sentiment.zip"))
   return
 end